1. Field of the Invention
The present invention relates to a process for preparing superconducting junctions of oxide superconductor such as Josephson Junction, more particularly to a process for preparing a superconducting junction by depositing successively a first oxide superconductor thin film layer, a non-superconducting intermediate thin film layer and a second oxide superconductor thin film layer on a substrate in this order.
2. Description of the Related Art
In order to realize electronics devices such as Josephson element or superconducting transistors in which superconductor is combined with semiconductor, it is indispensable to deposit successively at least one thin film layer of superconductor and at least one thin film layer of non-superconductor on a substrate. Hereinafter, we will describe the present invention with reference to Josephson junction used for fabricating Josephson element but it is apparent that the process according to the present invention is applicable to the other type superconducting devices.
A variations of Josephson Junctions have been reported. Among them, the most simplest type is so-called tunnel Josephson Junction in which a non-superconducting thin film layer is sandwiched between a pair of superconductors. Tunnel type Josephson element is fabricated by processing microscopically a layered structure of 1st superconductor thin film layer/non-superconductor thin film layer/2nd superconductor thin film layer. In this Josephson Junction, thickness of the intermediate thin film layer of non-superconducting is determined by the coherent length of superconductors.
Although several ideas for fabricating the tunnel type superconducting junctions from metallic superconductors have been reported, there is little report describing how to fabricate the same from oxide superconductors because there remain following problems to be solved in order to prepare the tunnel type superconducting junctions having oxide superconductor thin film layers.
Firstly, in the case of tunnel type superconducting junctions having oxide superconductor thin film layers, the thickness of non-superconducting intermediate thin film layer must be reduced to several nanometers (nm) because of very short coherent length of oxide superconductor.
Secondly, it is difficult to deposit and/or laminate oxide thin films having good crystallinity successively. In particular, in the prior art, it is difficult to prepare an upper superconductor thin film layer having good crystallinity on a non-superconducting intermediate thin film layer due to very poor crystallinity of the later layer. In order to improve performance of superconducting thin film devices, it is preferable that all thin film layers are made of single crystals. In fact, if any one of the layers is of a polycrystal or amorphous film, the tunnel type Josephson Junction doesn't work stably. In other words, there is a strong demand to prepare the second or upper superconductor thin film layer of single crystal having good crystallinity on a non-superconducting intermediate layer. Deposition of oxide superconductor thin film layer of single crystal having good crystallinity is requested also in the field of superconducting transistor having an interface between oxide superconductor and semiconductor.
In fact, tunnel type superconducting junctions prepared from oxide superconductor by the conventional technique did not show desired electrical properties due to poor interface condition between the oxide superconductor thin film layer and the non-superconductor thin film layer.
Therefore, an object of the present invention is to solve the problems and to provide a process for improving crystallinity of the non-superconducting intermediate thin film layer and hence the second or upper superconducting thin film layer in order to permit high-temperature superconductors to be utilized in actual uses.